The invention relates to an electromagnetic shoplifting detection system of the high-frequency type. In known shoplifting detection systems, a transmitting coil of a transmitter or a transmitter/receiver generates a magnetic alternating field with a periodically varying frequency. This frequency is generally in the range of 1 to 10 MHz. To the articles to be protected so-called detection labels are attached. These labels each comprise a resonant circuit consisting of an air-core coil tuned with a capacitor. If such a label is carried into the magnetic alternating field of the transmitter coil, at those instants when the frequency of the alternating field is equal to the resonant frequency of the resonant circuit in the detection label, that resonant circuit will absorb energy and then oscillate. This oscillation can be detected by a receiver circuit, which is either connected to the transmitting antenna, as is the case in a so-called absorption system, or is coupled with a second (receiving) antenna, as is the case in a so-called transmission system. Such shoplifting detection systems are known inter alia from applicant's Netherlands patent applications 82.02951 (corresponding to U.S. Pat. No. 4,686,517) and 89.00658. The transmitting/receiving antennas of a complete shoplifting detection system can for instance form a row of upright panels or self-supporting antenna coil constructions, sometimes referred to as pillars. Heretofore, the pillars have mainly been used in clothes stores, with the pillars being positioned directly in front of the exits. In such surrounding, the pillars are mounted directly on the floor, the direct vicinity of each pillar being free of obstacles. A new application of shoplifting detection systems of the high-frequency type concerns their use in supermarkets, where the articles purchased are paid for at so-called checkout units. Checkout units are normally constructed with metal beams, metal or wooden surfaces, provided with a conveyor belt, an (electronic) cash register and sometimes a barcode scanner. A cashier is seated in such a checkout unit, the customers pass in front of the unit and deposit the articles to be paid for on the conveyor belt. The belt carries the articles to the cashier, who enters them in the cash register using a barcode scanner, if provided, and finally payment is effected. The articles have meanwhile passed on to a location on the checkout unit behind the cashier, in the direction of the exit of the supermarket, i.e. a separation occurs of the stream of articles to be paid for and the customers involved. In a typical supermarket situation a series of these checkout units are lined up in a row and the customers pass between the checkout units in the direction of the exit. This system is sometimes referred to by the term "checkout system". To check whether customers who pass the checkout unit take out articles without depositing them on the belt, i.e. without paying for them, the customers must pass a detection field. For that purpose, one or two detection pillars are arranged in the passage which is formed between two checkout units and which the customers pass through.
Since the interspace is small with a view to efficient use of space, the pillars are arranged as close to the checkout units as possible; accordingly, in practice, the pillars are affixed to the checkout units. The detection pillars generate a magnetic alternating field on both sides. Similarly, the detection pillars have a sensitivity area extending on both sides of the pillar. The field on one side covers the passage the customers pass through; the field on the other side extends into the checkout unit. There are many conductors in the checkout unit. In the first place, metal beams, which form the mechanical structure of the checkout unit, can be conductors. In the second place, there may be disposed in the checkout unit many electricity lines, such as lines for the electricity supply to the checkout unit, for the conveyor belt, the cash register, the scanner, etc. In addition there are lines present for data transfer for the cash register, the scanner, the cash register computer system, etc. Thus a checkout unit can contain a whole system of cable ducts with cables, which, in addition, pass from the checkout unit and in many cases continue via the ceiling into other checkout units and which, moreover, provide connections with electrical and electronic apparatus arranged at other locations within the building of the supermarket. It is known from the general theory of the Electromagnetic field that a magnetic alternating field induces voltages, and hence currents, in all conductors disposed in that field. Conversely, conductors that carry an alternating current will generate a magnetic alternating field. In this way, the alternating field of the pillar, which penetrates into the checkout unit, will induce currents in the framework of the checkout unit, and in all cables disposed therein. These currents can travel form one checkout unit to another along the cable connecting the checkout units. There, these currents can induce voltages in the pillars mounted on the checkout units. In an absorption system, in which the transmitting and receiving antenna are shared, these parasitically coupled transmitted signals are superposed on the system's own transmitted signal. In a transmission system, the receiving antenna receives the directly coupled transmitted signal from the corresponding transmitter pillar, onto which the parasitically coupled transmitted signals of the other pillars are superposed. These parasitic signals have traversed signal paths which have a different length from the signal which the pillars are normally excited with. The signal paths in question are long relative to the wave length of the transmitted signal. With the widely used fundamental operating frequency of 8.2 MHz, where the transmission frequency in fact sweeps between 7.5 and 8.9 MHz, the wave length varies between 40 and 33.7 m. Across this distance, apart from a number of effects which make this distance shorter, there occurs a phase rotation of 360.degree. over the signal path. Depending on the phase difference between the transmitted signal proper and the parasitic signal, the two signals will reinforce or oppose each other.
As this phase difference is frequency-dependent, during the frequency sweep a change will occur from addition to subtraction of the signals and vice versa. It is further of importance that the parasitic signal paths consist of just any conductors, which are not intended to transport high-frequency signals and hence exhibit an altogether unpredictable behavior. As a result, impedances of signal sources do not correspond at all with characteristic impedances of the (parasitic) signal paths. Further, these characteristic impedances vary strongly along the course of a signal path. The signal paths are not characteristically terminated at all. The result is that along such a parasitic signal path, there are many points of reflection where the signal is reflected. The cables extending further into the building, too, exhibit points of reflection at greater distances, so that also signals with a great delay are reflected and exert their influence in the receiver. Therefore, in the cables of the checkout units, a multiplicity of standing waves arise, which standing wave pattern varies strongly with the variation of the frequency during the frequency sweep. Thus, the pattern of parasitic signals which a) go from one transmitter to one receiver via different paths, and b) go from different transmitters to one receiver, will exhibit a very erratic and unpredictable amplitude and phase behavior. In the radio communication technique this phenomenon is known by the name of multipath propagation. In the radio communication technique, it is also known that multipath propagation can give rise to serious signal distortion, particularly with frequency-modulated signals. In shops, multipath propagation can also be caused because the transmission field that extends into the checkout unit also induces currents in the metal framework. Due to its large dimensions, the framework will operate as an effective antenna, which in turn can induce currents in the framework of the adjacent checkout units, so that the pillars mounted thereon or adjacent thereto thereby receive a parasitic signal. These high-frequency currents in the framework are also responsible for the fact that false detection can happen when labels are disposed in certain locations of the checkout unit where an inductive coupling with the frame can occur. Finally, there is yet another coupling mechanism that can cause parasitic currents, which do not result from the magnetic alternating field generated by the pillars, but are caused by capacitive coupling. The parts of the transmitting antenna carry a high-frequency voltage.
On account of the area of the antenna parts a certain capacitance is present relative to the free space. As a result, a high-frequency current will flow to the free space (dielectric displacement flow). When an electric conductor of sufficient size is disposed in the vicinity of an antenna part, as a result of the capacitance from antenna part to conductor, an additional capacitive current will flow to this conductor. This current will continue into that conductor. When the conductor is connected to the above-mentioned cables in the checkout unit, this current will contribute to the parasitic couplings and hence to the multipath propagation effect. As the voltages on the antenna are symmetrically distributed, the capacitive currents will compensate each other due to the capacitances to the free space of the separate parts of the antenna. However, if the surroundings of the pillar are asymmetrical as far as the presence of electric conductors is concerned, the currents of the different parts will no longer compensate each other and a part of the total current will be compensated by a current coming from the feeder cables. These are common mode currents, which, because coaxial cables are involved here, are referred to as sheath currents here. The fact is that these high-frequency currents travel over the outside of the coaxial sheath. In reciprocal direction it is possible that these sheath currents induce signal voltages in receiving antennas which are also arranged in electrically asymmetrical surroundings, and thus contribute to the parasitic couplings. A solution for the effects of sheath currents has previously been described in applicant's Netherlands patent application 90.00377, which has been withdrawn prior to publication thereof.